1. Field of the Invention
The present invention relates generally to a gas turbine engine and, more particularly, to a variable contour annular air inlet for an aircraft engine nacelle.
2. Description of the Prior Art
An aircraft engine nacelle is typically a circumferentially extending housing for an aircraft engine, such as a gas turbine engine, which has an annular air inlet. Aerodynamic drag due to freestream airflow over the engine nacelle reduces somewhat the propulsion system thrust output. Any reduction in this aerodynamic drag can result in an increase in propulsion system thrust output during takeoff and low-speed operation and in a significant saving in the amount of fuel consumed during high-speed cruise operation. A desirable nacelle design, therefore, is one that provides a lightweight housing for the aircraft engine while producing relatively low aerodynamic drag.
The contour of the annular air inlet of the nacelle greatly affects aerodynamic drag and air flow characteristics. However, selection of the proper inlet contour is difficult in that different configurations are desirable for different operating conditions of the aircraft. For instance, during high-speed flight or cruise operation of the aircraft, an inlet having a thin sharp contour of reduced diameter is desirable to minimize aerodynamic drag. However, for takeoff or landing and low-speed operation of the aircraft, an inlet having a fat blunt contour of increased diameter is ideal to prevent flow separation and avoid aerodynamic drag increases which will reduce thrust during operation at high angles of attack.
One approach to nacelle air inlet contour selection is to design the inlet with a fixed configuration to serve the most critical operating condition of the aircraft, such being takeoff and landing. As a result, a performance penalty will be incurred at high-speed cruise operating conditions due to an excess amount of drag. This prior art approach is exemplified by U.S. Pat. No. 3,765,623 to Donelson et al.
Another approach to nacelle air inlet contour selection is to provide means for changing or varying the configuration of the inlet in accordance with the particular operating condition. This prior art approach is represented by U.S. Pat. No. 3,222,863 to Klees et al and U.S. Pat. No. 3,664,612 to Skidmore et al. The variable inlet contour approach offers more versatility and flexibility than the fixed inlet contour approach, but at the expense of more weight and mechanical complexity.
Still, the variable inlet contour approach appears to hold out more promise for achieving reduction of aerodynamic drag than the fixed inlet contour approach. Consequently, a need still remains for an improved way to provide a variable contour nacelle inlet while minimizing the above-described problems.